Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to a planetary gear train of an automatic transmission for a vehicle that improves power delivery performance and fuel economy by achieving ten forward speeds and widening speed ratio span (or gear ratio span), and secures linearity of step ratios.
Description of Related Art
Generally, an automatic transmission achieving more speed stages has been developed to enhancing fuel economy and optimizing drivability.
Such an automatic transmission achieving more speed stages is necessary to maximize power performance and driving efficiency according to downsizing of an engine. It is desperately demanded to develop high efficiency multiple-speeds transmissions having excellent linearity of step ratios that can be used as an index closely related to drivability including acceleration before and after shift and rhythmical engine speed to secure competitiveness of automatic transmission field.
However, in the automatic transmission, as the number of speed stages increase, the number of internal components increase, and as a result, mountability, cost, weight, transmission efficiency, and the like may still deteriorate.
Accordingly, development of a planetary gear train which may achieve maximum efficiency with a small number of components may be important to increase fuel efficiency enhancement effect through the multiple-speeds.
In this background, an eight-speed automatic transmission has been introduced recently and a planetary gear train for an automatic transmission facilitating more speed stages is under investigation.
However, since a conventional eight-speed automatic transmission has gear ratio span of 6.5-7.5 (gear ratio span is an important factor for securing linearity of step ratios), improvement of power performance and fuel economy may not be great.
Furthermore, even though an eight-speed automatic transmission has gear ratio span greater than 9.0, it is hard to secure linearity of step ratios. Therefore, driving efficiency of an engine and drivability of a vehicle may deteriorate, and thus, development of high efficiency automatic transmissions which achieve at least nine forward speeds is necessary.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and may not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.